When a ray of light travels from an optically denser medium to an optically rarer medium and is incident at an angle greater than the critical angle for the two media, the ray totally reflects back into the medium, obeying the laws of reflection. This phenomenon is known as the total internal reflection. Following are the two conditions for the total internal reflection to take place:
The angle of incidence in the denser medium must be greater than the critical angle for that pair of media.
- The ray of light must travel from a denser medium into a rarer medium.
The critical angle is that angle of incidence in a denser medium, for which the angle of refraction in the rarer medium is 90 °.